1. Field of the Invention
Technical Field
The present invention relates to electrohydraulic projectors, particularly those utilizing an electrical plasma in a liquid to create acoustic, pressure, and shock waves, and methods for efficiently coupling the electrical current to the plasma.
2. Background Art
The underwater plasma (10) physical processes at issue are shown in FIG. 1. When high voltage is impressed across two electrodes (11) immersed in water (12) or some other liquid, and the electric field (voltage divided by the electrode separation and modified for the shape of electrodes) is above the breakdown electric field of the water (12), then a conducting plasma channel (10) forms between the two electrodes (11). Especially if significant current is passed through the conducting channel (10), a number of important events occur. A zone of steam or vapor is formed around the plasma channel (10), and this bubble (13) of steam (14) propagates outward from the channel (10) at a rate that is a function of the power deposited by the electrical current in the channel (10). Power is conducted from the channel (10) to the steam (14) via thermal conduction and by thermal radiation. A significant portion of the thermal radiation is trapped in the water (12) and produces ablation of the bubble wall (13), thus adding additional steam (14) to the bubble (13).
An underwater plasma of this type can be controlled to have useful characteristics. High power levels in the underwater plasma (10) will produce very strong pressure waves (15) as the steam bubble (13) expands against the water. Lower power levels in the plasma will produce acoustic waves (15) to produce sound for particular applications. By modifying the temporal behavior of the power deposition in the plasma (10), and taking into account the inertia of the moving water, the acoustic spectrum can be modified.
There are a number of situations where it is desirable to create intense shock waves or high pressure waves under water. These applications include: 1) crushing rock for mining and drilling, 2) obstacle clearing where such high pressure waves are created to remove or destroy obstacles such as reefs, old concrete construction, or similar objects, and 3) where it is desired to create high energy acoustic waves for undersea oceanographic mapping. Using electrical sparks underwater or underwater plasmas for the creation of pressure waves has been attempted. However, it has not heretofore been possible to create efficient high energy waves. The primary reasons for this are the difficulty with efficiently loading energy into salt water and the difficulty of efficiently loading electrical energy into any type of underwater plasma.
Most drilling techniques utilize mechanical fracturing and crushing as the primary mechanism for pulverizing rock. A new approach utilizing underwater sparks called spark drilling, was introduced in the 1960's and mid 1970's. Maurer (Maurer, W. C., "Spark Drilling," Proc. 11th Symposium on Rock Mechanics, University of California, Berkeley, Jun. 16-19, 1969) described earlier work on spark drilling, including some high pressure chamber testing of the spark apparatus. Sandia National Laboratories picked up the concept and began to pursue it aggressively. Alvis, R. L., "Improved Drilling--A Part of the Energy Solution," Sandia Laboratories Report No. SAND-75-0128, Albuquerque, N. Mex., March 1975; Newsom, M. M., "Program Plan for Improving Deep Drilling," Sandia National Laboratories Report No. SLA-74-0125, Albuquerque, N. Mex., May 1974; and Newsom, M. M., "Drilling Research at Sandia National Laboratories," Sandia Laboratories Report No. SAND-76-5194, Albuquerque, N. Mex., March 1976. Sandia primarily focused on preventing flashover of insulators and were able to measure reasonable drilling rates. A major thrust of the Sandia work was controlling electric fields in an attempt to overcome the spark-over problem. Wardlaw (Wardlaw, R., et al., "Drilling Research on the Electrical Detonation and Subsequent Cavitation in a Liquid Technique--Spark Drilling," Sandia National Laboratories Report No. SAND-77-1631, Albuquerque, N. Mex., 1978) conducted tests of the 20 cm drill with a nominal power output of around 25 kW and demonstrated high peak pressures in the 500-1000 mega Pascal range during the testing. However, electrode life and the capability of efficiently loading energy into the water caused Sandia to discontinue work on the drills.
Other research was conducted with other variations of spark drills including utilizing sparks to enhance cutting power of low pressure water jets. These early experiments are well summarized in Maurer's book. Maurer, William C., Advanced Drilling Techniques, Petroleum Publishing Co., Tulsa, Okla., 1980.
The common problem in all of these spark approaches is that they dealt with the mechanics of the shock wave or insulator flashover problem but did not address the primary issue, which is control of the underwater plasma that creates the shock wave. For the last decade, Tetra Corporation has focused on understanding and controlling this plasma for spark drill technology development. U.S. Pat. No. 4,741,405, to Moeny et al., taught a technique for controlling power to the arcs through the use of pulse forming lines. This produced a substantial enhancement of the drilling process.